1. Field of the Invention
The invention relates to an evacuated sheet material for thermal insulation, the sheet material comprising at least one flat core made of an open-pored material, a first barrier film in large-area contact with a first main surface of the core and having at least one sealing layer facing the core, a second barrier film encasing the core on its other main surface and having at least one sealing layer and a fully circumferential sealing seam along which the two barrier films are sealed to each other, wherein the volume between the two barrier films and the fully circumferential sealing seam is evacuated.
2. Description of the Prior Art
Evacuated insulating materials achieve thermal conductivities five to twenty times lower than those of ventilated, conventional insulating materials. They can be used, for example, to make very compact, high-insulating shipping containers for temperature-sensitive goods, high-insulating refrigeration and freezing equipment, or ultra-slim insulation systems for the building industry.
Suitable materials for the core of the evacuated insulation panels are compression resistant materials in the form of powder boards, powder fills, open-pore foams or glass fiber materials. In particular, to reduce dust, insulating cores consisting of powder boards or loose powder are usually wrapped in an air-permeable nonwoven polyester fabric of the kind described, for example, in DE 100 585 66. This keeps dust from being released during the evacuation process in the vacuum chamber and contaminating both the sealing seams and the vacuum chamber itself.
Core panels made of microporous silicic acid powder have a very fine pore structure and permit relatively high gas pressures without any consequences from the thermal conductivity of the residual gas. Thus, with these microporous materials, a vacuum of only 1 to 10 mbar is sufficient to lower the thermal conductivity to 0.004 to 0.005 W/mK. Envelopes of special high-barrier films with only an ultra-thin, vapor-deposited aluminum coating ensure that the gas pressure in the core material will increase only about one mbar per year. However, the manufacturing processes available to date for making powder-filled vacuum insulation panels are relatively elaborate and cannot be automated completely. One method that comes relatively close to meeting the requirements for automation is described in DE 10 2005 045 726, and includes the following method steps: a powder is filled into a high-barrier film bag, the shell of which has regions that will subsequently form the main surfaces of the finished flat insulation element, while a still-open region of the bag will subsequently form part of the perimeter of the finished flat insulation element; a filter material that is permeable to air, but impermeable to powdery dust, is fixed near the opening on the inner face of the film bag so that the inside of the bag is sealed dust-tight but still allows air to escape; the interior is evacuated; and finally, the bag is sealed in the evacuated state, the filter material ultimately being completely covered externally by the folded-together bag opening and thus being disposed entirely inside the bag, i.e., entirely inside the film envelope of the finished thermal insulation element.
During evacuation, the fine powders present can be completely retained in the bag, even under high gas flow rates, by the filter material disposed in the bag opening, so the evacuation chamber and the sealing seams do not become contaminated. The imperviousness of the finished thermal insulation element is still optimal, the filter material ultimately is completely covered by the film envelope. However, this method has the disadvantage that the evacuation process takes a relatively long time, since the suctioning can be done only through the narrow opening where the filter fabric is situated, which has a relatively small flow cross section. In addition, one-sided evacuation makes for a rather uneven distribution of powder over the surface of the panel.